Optical fiber cables typically include optical fibers or ribbons provided in buffer tubes. For water blocking purposes, the buffer tube includes either a filling gel or a water-swellable material, such as a water-swellable yarn. Water-swellable yarns typically comprise granular powder glued to a polyester carrier yarn or polyacrylate filaments that are bundled onto polyester carrier yarns. However, due to their irregular geometry and granularity, the particles of powder on the carrier yarn in the first instance can damage the performance of the optical fibers. Also, with respect to the water-swellable filaments spun with polyester yarns, too much bulk must be added inside the tubes.